Our Black Hole's Magnetic Field Has Been Studied For The First Time

Artist's concept of the black hole at the center of our galaxy, surrounded by a hot disk of accreting material. Blue lines trace magnetic fields. M. Weiss/CfA

Astronomers, for the first time, have observed the expected magnetic field around the event horizon of Sagittarius A*, the supermassive black hole at the center of the Milky Way. The results are published in the journal Science.

The international team found that the magnetic field of the black hole is very intricate and it varies from region to region of the object. Some parts of the event horizon (the boundary from which nothing, even light, can escape) are messy, with wobbly lines being all jumbled up, while other areas have the magnetic structure organized in patterns. The magnetic field also fluctuates wildly, changing on a scale of about 15 minutes.

"Understanding these magnetic fields is critical. Nobody has been able to resolve magnetic fields near the event horizon until now," said lead author Michael Johnson in a statement. "Once again, the galactic center is proving to be a more dynamic place than we might have guessed. Those magnetic fields are dancing all over the place."

The supermassive black hole has a mass equivalent to 4 million Suns, and the event horizon extends for 13 million kilometers (about 8 million miles), 40 times the distance from Earth to the Moon. Sagittarius A* is 25,000 light-years away, making the event horizon's apparent size, gravitational effects included, 50 micro-arcseconds of a degree. By comparison, from Earth a golf ball on the Moon appears to be about 15 micro-arcseconds.

This extraordinary observation was possible by using the Event Horizon Telescope, a global network of radio telescopes that link together and act as a single Earth-sized telescope.  

"The only way to build a telescope that spans the Earth is to assemble a global team of scientists working together," Shep Doeleman, co-author of the paper, said in the statement. "With this result, the EHT team is one step closer to solving a central paradox in astronomy: why are black holes so bright?"

Black holes can generate powerful jets, shooting particles to almost the speed of light. Electrons are accelerated in corkscrew orbits around the magnetic field lines produced by the black hole. Although the mechanisms to produce jets has long been understood, this is the first time we have a direct observation of the magnetic field.

"These magnetic fields have been predicted to exist, but no one has seen them before. Our data puts decades of theoretical work on solid observational ground," concluded Doeleman. 

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